Ring galaxies. II

Abstract

Dynamical problems concerning the formation and evolution of ring galaxies are discussed with the help of a simulation code for disk systems. The dynamical age of ring galaxies is found to be of the order of 100 million yr, in agreement with other estimates of age. On a longer time scale, the ring breaks up into about six smaller objects which subsequently collide and merge. It is thought that this system forms a galaxy with a gas-enriched active nucleus. The simulations are used to study the effect on a normal disk galaxy of a passage through its center by a second galaxy. The gas in the disk forms a ring, and the stellar core is distended. The galaxy-collision model for ring galaxies, therefore, has difficulty in rationalizing the RE galaxies, in which the original core is not in evidence. The difficulty is alleviated if the target core loses some mass to the intruding galaxy so that its remnant disperses. This process is simulated numerically for a pure population I target galaxy with the help of a simple model. The model is axisymmetric but has dispersion normal to the disk plane. It gives an impression of the three-dimensional aspects of the collision as seen in projection.